This invention relates to a method for the anticorrosive treatment of waste plastics treating equipment in which waste plastics are treated by supercritical water.
It is conventionally known that natural and synthetic polymeric compounds can be decomposed by means of supercritical or subcritical water.
Meanwhile, in equipment for the treatment of chlorine-containing waste plastics such as vinyl chloride resin and polychlorotrifluoroethylene, chlorine is released from such resins, and the equipment material (e.g., stainless steel) is heavily corroded by the chlorine. Although the degree of corrosion may vary according to the chemical composition of the equipment material and the Cl concentration in the aqueous solution, the treatment especially with supercritical water is carried out under high-temperature and high-pressure conditions. Consequently, when stainless steel is used as the equipment material, it is highly susceptible to pitting, crevice corrosion, stress corrosion cracking and other corrosion due to chlorine ion. As a countermeasure against it, nickel alloys having special chemical compositions have been used.
However, such nickel alloys are so expensive that the construction cost of the equipment is raised to an undue extent. Accordingly, in the case of equipment for the treatment of chlorine-containing waste plastics such as vinyl chloride resin and polychlorotrifluoroethylene, there has been developed a process which avoids the direct treatment of such resins with supercritical water. Specifically, according to this waste plastics treating process, such resins are first dechlorinated by pyrolysis at 300-350.degree. C. in a separate dechlorination apparatus, and the remaining dechlorinated resins are then treated with supercritical water.
As a result of dechlorination, this process permits relatively inexpensive stainless steel to be used as the equipment material. However, this process is more complicated and requires a greater number of system components, as compared with the process for treating waste plastics directly. As a result, this process still involves the problem that the construction cost of the system is very high.